12 MASS. EXPERIMENT STATION BULLETIN No. 283 



and ill air with a relative lumiidity of not less than 90 per cent at about 20° C. 

 I'nder these conditions, drops of water of guttation soon appeared on the margins 

 of the leaves. A good infection, including sporulation, resulted, and it was 

 (ionfined to the margins of the leaves where the water of guttation had stood, 

 although conidia had been appHed all over the surface of the leaves. No infection 

 resulted when cucumber plants were thus inoculated by placing dry conidia on 

 dry leaves if the inoculated plants were then grown in soil and air too dry for 

 water of guttation to appear. 



The relation of air temperature to infection of cucumber bj' P. cubensis has 

 been investigated by Hiura (loc. cit.) who concluded that the minimum tempera- 

 ture for infection is 10° to 15° C, the optimum about 20° C, and the maximum 

 at or not far above 30° C. The writer has secured infection of cucumber plants 

 inoculated at 10°C., but not below, and at 28° C, but not above; most complete 

 infection has been secured at temperatures between 16° and 22° C. These tem- 

 peratures at which infection is possible correspond fairly closely to the cardinal 

 temperatures for the germination of conidia, to which reference has already been 

 made. 



Infection through upper as compared with lower surfaces of the leaves is of 

 interest, as having a possible bearing on the use of fungicides. The observations 

 of the writer lead him to agree with earlier investigators (38, 42) that infection 

 by the germ tubes of the zoospores of this fungus is through the stomata. How- 

 ever, as has been recorded by others (38, 16), cucumber plants may be infected 

 by this fungus through both the upper and lower leaf surfaces. As determined 

 by the writer, there are two to three times as many stomata per unit area on the 

 lower surface as on the upper surface of cucumber leaves, but there is abundant 

 opportunity for infection through both surfaces except as they are protected by 

 fungicides. In fact, it is probable that more leaves are infected through the 

 upper surface, for the writer was able to prevent most (but not all) accidental 

 infection by applying a fungicide to the upper surfaces of leaves, but not the lower. 



The incubation period for downy mildew of cucumber may mean either the 

 interval of time between inoculation and sporvilation, that is, the appearance of 

 conidiophores and conidia on inoculated leaves, or it may mean the interval of 

 time between inoculation and the appearance of the yellowish leaf spots indica- 

 tive of infection. These leaf spots usually appear three to five days after inocu- 

 lation. With temperature and moisture conditions near the optimum, infection 

 of leaves usually ends in sporulation within six to nine days after inoculation. 

 In the case of cotyledons of cucumber seedlings, the incubation period may be 

 shorter, so that conidiophores and conidia are produced four to six days after 

 inoculation. If the incubation period is understood to end only with sporulation, 

 it may, as is described below, be prolonged in dry air almost indefinitely, within 

 the life of the plant of course. Even in moist air, the incubation period is affected 

 somewhat bj' temperature. The incvibation period ended in sporulation seven 

 days after inoculation at 20° to 28° C. and nine days after inoculation at 16° 

 to 20° C. 



These results may help to explain the observation of Hiura (loc. cit.) who 

 noted that the appearance of the disease in the field was hastened by high tem- 

 peratures up to 30° C, and delayed by low temperatures near the mininunn for 

 infection. 



